Permanent magnets are often used in large electrical machines such as motors or generators. Such an electrical machine comprises two basic components, namely a field for creating magnetic flux, and an armature for generating electromotive force and for carrying current crossing the field. The armature usually comprises conductive coils wrapped on a stator, while the field usually comprises magnets arranged on a rotor. The rotor can surround the stator, or vice versa, and the magnets and coils face each other across a narrow air gap. The established methods of loading or mounting permanent magnets onto the field (or ‘field structure’) of an electrical machine comprise various steps such as enclosing the individual permanent magnet poles in housings, and gluing the permanent magnet poles to the field (usually the rotor), wrapping the entire arrangement in fibreglass bandage or enclosing it in a vacuum bag, pumping resin into the bag and performing vacuum extraction to consolidate the permanent magnet poles to the rotor body. These methods are accompanied by various problems such as the extensive and therefore costly effort involved in securing the permanent magnets to the field. For example, when the magnets are glued to the field, the glue must be allowed to cure or set, adding to the overall assembly time. Furthermore, in case of a failure of a permanent magnet, the defective magnet must be removed and replaced, which is made difficult if a fibreglass or resin envelope must be opened and then resealed again, making repairs complicated and costly to carry out.
The permanent magnet poles—which can be several meters in length and correspondingly heavy—are usually already magnetized before they are mounted onto the field. Therefore, these can be strongly attracted to other permanent magnets of opposite polarity already in place on the field, or to other loose magnetic items such as tools or fasteners. Therefore, permanent magnets present a considerable safety hazard during the mounting procedure. For these reasons, handling of the permanent magnets requires special machinery and tools and very strict work-flow control to avoid potentially hazardous situations. An alternative approach, involving first loading the magnets onto the field and then magnetizing them, would avoid the hazardous manual handling but would be very costly and therefore impracticable to implement.
One possible solution involves arranging a number of holding or gripping elements onto an inner surface of the field, such that these can hold a magnet pole in place. The holding elements can be designed to allow the magnet pole to be pushed in from the side. The holding elements must be robust enough to withstand the strong magnetic forces acting on the permanent magnets. Such a design is therefore associated with a number of drawbacks, such as the need to attach many relatively robust and therefore heavy holding elements to the rotor surface that add to the overall weight of the rotor. Furthermore, the holding elements need to extend beyond the upper surface of the magnets in order to effectively hold them in place, and this might require a wider air gap.
For the above-mentioned reasons, the prior art methods of mounting permanent magnets to a field of an electrical machine are hazardous and difficult to carry out, present problems during maintenance, or involve costly machining steps.